The terms UE (User Equipment), terminal, handset, etc. may be used interchangeably to denote a device that communicates with the network infrastructure. The term should not be construed as to mean any specific type of device, it applies to them all, and the embodiments described herein are applicable to all devices that may use the embodiments to solve the problems as described.
Similarly, a base-station is intended to denote a node in the network infrastructure that communicates with the UE. Different names may be applicable, and the functionality of the base-station may also be distributed in various ways. For example, a radio head may terminate parts of the radio protocols and a centralized unit may terminate other parts of the radio protocols. This disclosure does not distinguish such implementations herein. The term base-station refers to all alternative architectures that can implement the embodiments described herein.
Third Generation Partnership Project (3GPP) describes a new radio (NR) interface for fifth generation (5G) networks. Terms for denoting this new and next generation technology have not yet converged, so the terms NR and 5G may be used interchangeably. Moreover, a base-station can be referred to as gNB instead of eNB. Alternatively, the term Transmission-Receive-Point (TRP) can also be used.
An NR radio slot consists of several orthogonal frequency division multiplexed (OFDM) symbols. The slot includes either 7 or 14 symbols with OFDM subcarrier spacing≤60 kHz and 14 symbols for OFDM subcarrier spacing>60 kHz. FIG. 1 illustrates a subframe with 14 OFDM symbols. In FIG. 1, TS and Tsymb denote the slot and OFDM symbol duration, respectively. In addition, a slot may be shortened to accommodate downlink/uplink transient periods or both downlink and uplink transmissions. Potential variations are illustrated in FIG. 2.
Furthermore, NR also defines mini-slots. Mini-slots are shorter than slots (according to current agreements, mini-slots include from 1 or 2 symbols up to the number of symbols in a slot minus one) and can start at any symbol. Mini-slots are used if the transmission duration of a slot is too long or the occurrence of the next slot start (slot alignment) is too late. Applications of mini-slots include, among others, latency critical transmissions (in this case both mini-slot length and frequent opportunity of mini-slot are important) and unlicensed spectrum where a transmission should start immediately after listen-before-talk succeeded (here the frequent opportunity of mini-slot is especially important). An example of mini-slots is illustrated in FIG. 3.
PDCCHs (physical downlink control channels) are used in NR for downlink control information (DCI), e.g. downlink scheduling assignments and uplink scheduling grants. The PDCCHs are in general transmitted at the beginning of a slot and relate to data in the same or a later slot (for mini-slots PDCCH can also be transmitted within a regular slot). Different formats (sizes) of the PDCCHs are possible to handle different DCI payload sizes and different aggregation levels (i.e., different code rate for a given payload size). A UE is configured (implicitly and/or explicitly) to monitor (or search) for a number of PDCCH candidates of different aggregation levels and DCI payload sizes. Upon detecting a valid DCI message (i.e., the decoding of a candidate is successful and the DCI contains an ID the UE is told to monitor) the UE follows the DCI (e.g., receives the corresponding downlink data or transmits in the uplink).
NR may include a broadcasted control channel to be received by multiple UEs. The channel has may be referred to as group common PDCCH. One example of information that might be put in such a channel is information about the slot format (i.e., whether a certain slot is uplink or downlink, which portion of a slot is uplink or downlink, information that can be useful in a dynamic time division duplex (TDD) system, etc.).
The downlink control information (DCI) carries several parameters to instruct the UE how to receive the downlink transmission or to transmit in the uplink. For example, the frequency division duplex (FDD) long term evolution (LTE) DCI format 1A carries parameter such as localized/distributed virtual resource block (VRB) assignment flag, resource block assignment, modulation and coding scheme (MCS), hybrid automatic repeat request (HARQ) process number, new data indicator (NDI), redundancy version and transmission power control (TPC) command for physical uplink control channel (PUCCH).
One of the key parameter for the UE to receive or transmit in the system is the size of the data block (called transport block size) to be channel coded and modulated. In LTE, this is determined as follows. The UE uses an MCS given by the DCI to read a transport block size (TBS) index ITBS from a MCS table. An example of the MCS table is shown in Table 1 below. The UE determines the number of physical resource blocks (PRBs) as NPRB from the resource block assignment in the DCI. The UE uses the TBS index ITBS and the number of PRBs NPRB to read the actual transport block size from a TBS table. A portion of the TBS table is shown in Table 2 below as an example.
TABLE 1LTE modulation and coding scheme (MCS) tableMCSModulationTBSIndexOrderIndexIMCSQmITBS0201212223234245256267278289291049114101241113412144131541416415176151861619617206182161922620236212462225623266242762528626292reserved304316
TABLE 2LTE transport block size (TBS) table (dimension is 27 × 110)NPRBITBS123456789. . .016325688120152176208224. . . 1245688144176208224256328. . . 23272144176208256296328376. . . 340104176208256328392440504. . . 456120208256328408488552632. . . 572144224328424504600680776. . . 632817625639/504600712808936. . . 71042243284725847128409681096. . . 812025639253668080896810961256. . . 9136296456616776936109612561416. . . 101443285046808721032122413841544. . . 1117637658477610001192138416081800. . . 1220844068090411281352160818002024. . . 13224488744100012561544180020242280. . . 14256552840112814161736199222802600. . . 15280600904122415441800215224722728. . . 16328632968128816081928228026002984. . . 173366961064141618002152253628563240. . . 183767761160154419922344279231123624. . . 1940884012881736215226002984 34963880. . . 204409041384186423442792324037524136. . . 2148810001480199224722984349640084584. . . 2252010641608215226643240375242644776. . . 2355211281736228028563496400845845160. . . 2458411921800240829843624426449685544. . . 2561612561864253631123752439251605736. . . 2671214802216298437524392516059926712. . .
The LTE approach has a few problems described in the following. A first problem is that the LTE TBS table was originally designed with specific assumptions on the number of resource elements (REs) available within each allocated PRB as well as the number of OFDM symbols for data transmissions. When different transmission modes with different amounts of reference symbol overheads were introduced later in LTE, it became difficult to define another TBS table to optimize for the new transmission modes. The 3GPP participants compromised to introduce a few new rows in the LTE TBS table to optimize for a few limited cases. The explicit TBS table approach, however, hinders continual evolution and improvement of the LTE system.
Another problem is that the existing approach of determining the data block size does not provide high performance operation with different slot sizes or structures. This is a known problem in LTE systems because a subframe in LTE may be of various sizes. A regular subframe may have different sizes of control region and thus leaves different sizes for the data region. TDD LTE supports different sizes in the downlink part (DwPTS) of a TDD special subframe. Various different sizes of subframe are summarized in Table 3 below.
The LTE MCS and TBS tables are designed, however, based on the assumption that 11 OFDM symbols are available for the data transmission. When the actual number of available OFDM symbols for physical downlink shared channel (PDSCH) is different than 11, the spectral efficiency of the transmission will deviate from those shown in Table 4 below.
First, the code rate becomes excessively high when the actual number of OFDM symbols for PDSCH is substantially less than the assumed 11 symbols. These cases are highlighted with bold type in Table 4. In LTE, the UE is not expected to decode any PDSCH transmission with effective code rate higher than 0.930. Because the mobile station will not be able to decode such high code rates, transmissions based on these bold type MCSs will fail and retransmissions will be needed.
Second, with the mismatch of radio resource assumption, code rates for some of the MCSs deviate out of the optimal range for the wideband wireless system. Based on extensive link performance evaluation for the downlink transmission as an example, the code rates for QPSK and 16QAM should not be higher than 0.70. Furthermore, the code rates for 16 QAM and 64 QAM should not be lower than 0.32 and 0.40, respectively. As illustrated with underlined type, some of the MCSs in Table 4 result in sub-optimal code rate.
Because data throughput is reduced when transmissions are based on unsuitable or sub-optimal code rates, a good scheduling implementation in the base station should avoid using any bold or underlined MCSs shown in Table 4. Thus, the number of usable MCSs shrink significantly when the actual number of OFDM symbols for PDSCH deviates from the assumed 11 symbols.
TABLE 3Available number of OFDM symbols for PDSCH (NOS) in LTENumber of OFDM symbolsfor control informationOperation mode1234FDD, TDDNormal CP13121110Extended CP111098TDD DwPTSconfigurations 1,8765normal CP6configurations 2,98767configurations 3,109878configuration 4111098TDD DwPTSconfigurations 1,7654extended CP5configurations 2,87656configuration 3
TABLE 4Code rate with different number of OFDM symbols for data transmission in LTEMCS indexAvailable number of OFDM symbols for PDSCH (NOS)(IMCS)Modulation13121110987650QPSK0.100.110.120.130.140.160.180.210.251QPSK0.130.140.160.170.190.210.240.280.342QPSK0.160.170.190.210.230.260.300.350.423QPSK0.210.220.250.270.300.340.390.450.544QPSK0.250.280.300.330.370.410.470.550.665QPSK0.310.340.370.410.450.510.580.680.816QPSK0.370.400.440.480.540.610.690.810.977QPSK0.440.470.520.570.630.710.810.941.138QPSK0.500.540.590.650.720.810.931.081.309QPSK0.560.610.670.730.810.911.051.221.461016 QAM0.280.300.330.370.410.460.520.610.731116 QAM0.310.340.370.410.450.510.580.680.811216 QAM0.360.390.430.470.520.580.670.780.941316 QAM0.400.440.480.530.580.660.750.881.051416 QAM0.460.500.540.590.660.740.850.991.191516 QAM0.510.550.600.660.740.830.951.101.331616 QAM0.540.590.640.710.790.881.011.181.411764 QAM0.360.390.430.470.520.590.670.790.941864 QAM0.390.420.460.500.560.630.720.831.001964 QAM0.430.460.510.560.620.690.790.931.112064 QAM0.470.510.550.610.680.760.871.011.222164 QAM0.510.550.600.660.740.830.951.101.322264 QAM0.550.600.650.720.790.891.021.191.432364 QAM0.590.640.700.770.860.961.101.291.542464 QAM0.640.690.750.830.921.041.181.381.662564 QAM0.680.740.800.880.981.101.261.471.772664 QAM0.720.780.850.941.041.171.341.561.882764 QAM0.750.810.890.981.091.221.401.631.952864 QAM0.880.951.041.151.271.431.641.912.29
As mentioned above, the slot structure for NR tends to be more flexible than LTE with much larger range of the amount of allocated resources for UE to receive or transmit. The benefit of designing a TBS table diminishes significantly.
To solve these problems, suggestions have been made to determine the TBS through a formula instead of a table. One example determines the TBS as follows:
  TBS  =      A    ×          ⌈                                    N            PRB                    ·                      N            RE                          DL              ,              PRB                                ·          v          ·                      Q            m                    ·          R                A            ⌉      where                ν is number of layers the codeword is mapped onto        NREDL,PRB is the number of REs per PRB per slot/mini-slot available for carrying the PDSCH.        NPRB is the number of allocated PRBs        modulation order, Qm, and target code rate, R, are read from a MCS table based on IMCS signalled in the DCI.        an example value of A is 8, to ensure that TBS is a multiple of 8.        
Here NPRB, NREDL,PRB, ν, Qm, R are signaled through DCI or are configured through higher layers.
A problem with existing solutions is that the LTE transport block size (TBS) table is designed so that when code block segmentation is performed, all code blocks have the same size after segmentation. This property is desirable since it makes implementation easier. When a formula such as the one described above is applied, however, this property is not necessarily fulfilled.